The separation of molecules is effected to a large extent with the aid of matrices which have connected thereto ligands which interact with the molecules concerned. These ligands may be ionic, hydrophobic or affinity ligands. Electrically neutral matrices of mutually different porosity are used when separating molecules in accordance with size as in gel filtration. These matrices are normally spheroidal in shape, in order to afford good flow properties. The flow properties of the separation system are also determined by the size of the particles present; the smaller the particle the higher the pressure drop, which results in a lower rate of flow. It is desirable in industrial applications to achieve high rates of flow, so that the molecules can be separated quickly. Another important parameter with regard to the particles used is the total specific surface area presented by the particles. The larger the specific surface area, the more quickly the molecules are able to penetrate the matrix and interact with the ligands. This specific surface area can be increased by reducing the sizes of the particles.
This antithesis is usually solved by taking a middle path, i.e. by using a relatively large particle size which is not optimum with regard to either the flow properties of the separation system or the specific surface area.
Animal cells have the capability to transform or produce complex compounds such as viral vaccines, immunochemicals, hormones or enzymes. The majority of animal cells are anchorage-dependent and thus demand a surface for their growth. Small beads (microcarriers) have been used to provide the necessary surfaces for anchorage-dependent cell growth.
Since the first use of these microcarriers for cell culture in 1976, a number of different materials have been employed for their preparation. These include dextran, gelatin, polystyrene and polyacrylamide and have, despite their different structures and composition all proven successful, to various extents, for cell culture. These microcarriers share, however, the common feature that only the surface area is utilized for cell growth, which implies a number of drawbacks. First, the cells are subjected to mechanical stress both by the mixing system in the reactor and by the motion of the beads in the medium. Second, in order to provide a large surface area the bead size has to be as small as possible. But, in order to achieve good growth a minimum number of cells are required on each bead. Thus, as the bead size decreases the number of cell doublings that can be achieved is reduced and a large number of transfers is required to reach the final production scale.
In order to increase the available surface area for cell growth, attempts have been made to provide porous microcarriers. It is known that macroporous matrices, e.g. collagen sponge, can be prepared by a freezing procedure, U.S. Pat. No. 4,412,947 (Chioca). The procedure involves dispersion/dissolution of collagen in dilute organic acid and a subsequent temperature reduction to -60.degree. C. The frozen dispersion is thereafter freeze-dried. It is also known that particles of collagen are suitable for the culture of animal cells, U.S. Pat. No. 4,565,580 (Miyata et al.). It is also known that animal cells can be entrapped and cultivated in beads of collagen, U.S. Pat. No. 4,647,536 (Mosbach and Milsson). A collagen sponge, containing heavy particles for density increase, has also been used for the culture of animal cells, International Patent Application PCT/US86/00600 (Verax Corp.). However, the microsponge produced by this method will have a pore volume between 70 and 98%. Since the mechanical stability decreases as pore volume increases, it will be expected that such highly porous microsponges will have a limited stability. The method of producing these porous microsponges is quite specific for proteins (collagen) and to our knowledge this method has not been successfully used for other polymers like dextran, poly-acrylamide, etc.